The present invention relates to a charge control circuit for a battery charger and a battery charger including such a charge control circuit.
These days many types of electronic devices use rechargeable batteries, such as a lithium ion battery. For safe charging of such a lithium ion battery, the charge current must be controlled by monitoring the temperature of a battery pack (for example, refer to Japanese Laid-Open Patent Publication No. 2003-199262, page 1 and FIG. 1). Japanese Laid-Open Patent Publication No. 2003-199262 discusses a technique for charging, discharging, and recharging a battery in an environment in which thermal conditions change readily. A battery charger, which charges a battery, includes a charging circuit and a temperature sensor. The charging circuit has a charge current output coupled to a battery. The temperature sensor detects the temperature of the battery. The temperature sensor and the charging circuit are used to set the charge current in accordance with the battery temperature.
A structure for estimating temperature will now be discussed with reference to FIGS. 7A to 7C. As shown in FIG. 7A, a battery pack 10 includes a battery cell CL1 and a thermistor TH1. The thermistor TH1 has a negative temperature coefficient (NTC) and measures the temperature based on the resistivity, which varies in accordance with the temperature. A battery charger 40 performs charge control using a plurality of temperature threshold values for the charge current, as shown in FIG. 7B, and the charge voltage, as shown in FIG. 7C. For example, the current or voltage is restricted in the low temperature range of temperatures T1 to T2, the standard temperature range of temperatures T2 to T5, and the high temperature range of temperatures T5 to T6. Temperatures T3 to T4 is the optimal temperature range for charging.
To estimate the resistance of the thermistor TH1, which is used to execute control that is in accordance with the temperature, the battery charger includes a resistor group RG1 that corresponds with the temperature threshold value. The resistor group RG1 may be of a series type or a parallel type. As shown in FIG. 8A, a series type includes a plurality of resistors R1 to R4, which are connected in series. A group of switches (switches SW1 to SW4) supplies voltage to each connection node of the resistors. As shown in FIG. 8B, a parallel type includes resistors R91 to R94, which are connected in parallel, and switches SW1 to SW4, which supply the resistors with voltage.
In the battery charger 40, reference voltage V0 is supplied to the thermistor TH1 via the resistor group RG1. In this case, the voltage at an external terminal TM2 (voltage between the two terminals of the thermistor TH1) is determined by the combined resistance of the resistors R1 to R4 or the resistances of the resistors R91 to R94 and the resistance of the thermistor TH1. A comparator CP1 compares the voltage at the external terminal TM2 with a temperature measurement reference voltage VREF0 to estimate the temperature threshold value of the battery pack 10.
Referring to FIG. 8C, during a temperature scan that sequentially closes the switches SW1 to SW4, the resistor group RG1 is connected to the thermistor TH1 to detect the temperature threshold value. To perform charging, the battery pack 10 must be attached to the battery charger 40. When charging starts in a state in which the battery pack 10 is not attached, a large load is applied to the battery charger 40. Thus, various ways to check that the battery pack 10 has been attached to the battery charger 40 have been discussed (refer to, for example, Japanese Laid-Open Patent Publication No. 2007-159292, page 1, FIGS. 3 and 4). Japanese Laid-Open Patent Publication No. 2007-159292 describes a technique using a mechanical switch, which is arranged in a housing of the battery charger 40. Referring to FIG. 9A, when a mechanical switch 500, which includes a movable contact, is pushed and the attachment of the battery pack 10 is thereby detected, a switch 501 closes. The structure shown in FIG. 9A requires a mechanical switch. This may results in problems concerning the mechanical strength and reliability.
Japanese Laid-Open Patent Publication No. 2007-159292 also describes a method for detecting attachment by measuring a terminal voltage of the thermistor TH1, as shown in FIG. 9B, which uses the thermistor TH1 in the battery pack. The thermistor TH1 is connected to ground in the battery pack 10, and the voltage goes lower than the reference voltage when the battery pack 10 is connected. Accordingly, connection of the battery may be detected by comparing the voltage obtained by subtracting an error voltage V5 from the reference voltage V0 with a threshold value. The output of the comparator is HIGH when the battery is not connected and LOW when the battery is connected.
Generally, a large capacitor is connected to the battery charger to prevent ESD stress. The system in FIG. 9A uses a capacitor of approximately 0.1 μF. This increases the CR time constant and requires a long time for a single cycle of a temperature scan, which sequentially switches the resistor connected to the thermistor TH1 for comparison with a threshold value.
Accordingly, a delay in detection would occur when the battery attachment detection is performed for each cycle of the temperature scan. Further, since the resistance of the thermistor varies with temperature, the temperature estimation accuracy decreases as the scan time decreases to perform detection within a short period of time.